The invention relates to a safety circuit for a household current powered chimney fan, which generates a signal when the ventilator fails to rotate.
In order to prevent a smoke downdraft and in order to improve the draft in general in a furnace chimney, fans or smoke extractors are used. In conjunction with the extraction of flue gases, especially from an open fireplace, safe combustion may be dependent on the ventilator actually turning and exhausting the gases. A natural draft that occurs when the ventilator does not rotate is not always sufficient, especially with fuel having a high moisture content. There may be several reasons for the lack of rotation, for instance an overheating safety fuse in the motor may have cut out, or the motor circuit may be dead for other reasons. Finally, a blockage may occur after extended use due to deposits of soot, ash, and tar, which occur most frequently when the fuel is not dry. Furthermore, a strong frost may cause condensed water to freeze preventing restarting of the motor after an intended stoppage.
It is known to provide alarms for indicating when a ventilator of the above-mentioned type fails to rotate. They are most frequently based on a magnetic detection, e.g. by means of a Hall element and a magnet fitted to the shaft of the ventilator. In U.S. Pat. No. 5,513,979 one such alarm for a DC motor is described. However, due to its simple construction, a chimney fan is preferably used with a single-phased asynchronous motor supplied by household current of the type using an auxiliary winding with a capacitor. Furthermore, a Hall element-based detection of movement may fail. See, for example, DE 101 59 033, which describes an alarm that provides an “emergency function”, i.e. a minimal regulation within a safe range, in case the rotation information from the Hall element is not received.
Thus, there is a need for a robust solution to the described problem, which may also form the basis for a differentiated display of a cause for the lack of rotation.